Sensors are devices that sense and respond to physical phenomena. They are generally characterized by an effective range in which they can detect those phenomena. If the sensor is not properly located relative to a target, the sensor may not respond despite the existence of the phenomena the sensor is designed to respond to. Depending upon the associated design, an operator might not be able to determine the difference between a failed scan and a successful scan which produced a negative sensing result. In certain applications such as in security scanning or medical diagnostic scanning, it is crucial that these two be distinguished. In short, there is a critical difference between the statements: there is no weapon located on the target; and I was not able to scan the target because they are out of range. As such, if the device does not have a way to distinguish between these two scenarios, careful attention must be paid to the training of operators as to the proper operation of the device to assure the target is always in range when a scan is conducted. However, training personnel in the proper usage of new equipment carries its own costs and introduces another potential source of error in the form of administrative oversight and human error.